In the field of video compression, communication, decompression, and display, there has been for many years problems associated with supporting both interlaced content and interlaced displays along with progressive content and progressive displays. Many advanced video systems support either one or the other format. As a result such devices as deinterlacers became an important component in many video systems. Deinterlacers convert interlaced video content into progressive video format.
Generally, video is filmed at 24 frames per second (fps). Video frames are converted to NTSC video using the telecine process, during which the rate of the video must be modified for playback at 29.97 fps. During the telecine process, 12 fields, which are equivalent to 6 frames, are added to each 24 frames of video so that the same images that previously made up 24 frames of video, now comprise 30 frames of video. Without the additional fields, each frame of the 24 frames is represented with 2 fields in interlaced format. However, the addition of 12 fields means that half of the frames are represented with 3 fields instead of 2. The video is then displayed in an alternating manner, where one frame is displayed using 2 fields, then the next using 3 fields, and so on. This process is called the 3:2 pull-down.
With video in interlaced format, where each frame is represented using 2 fields, one for the top lines (top field) and the other for the bottom lines of the frame (bottom field), the 3:2 pull-down is done such that the display order is a top field followed by a bottom field. An exemplary order of display would be: top field of frame A, bottom field of frame A, top field of frame B, bottom field of frame B, top field of frame B, bottom field of frame C, top field of frame C, bottom field of frame D, top field of frame D, bottom field of frame D, and so on. So the first group of five fields has 2 bottom fields and 3 top fields, the next group of five fields has 2 top fields and 3 bottom fields, alternating, and so on.
The input to a de-interlacing device may have a video input in interlaced format as specified hereinabove. When de-interlacing interlaced content that has also been formatted for 3:2 pull-down, reverse 3:2 pull-down must be performed so as to identify the fields that make up each frame of the progressive content and to use the correct 2 fields or 3 fields that represent each frame. A de-interlacer may, however, not have the capability to determine whether the input is in the usual interlaced format (i.e. 2 fields per frame) or in a 3:2 pull-down format. Hence, a de-interlacer may require a method for detecting 3:2 pull-down.
3:2 pull-down detection requires a matched filter to detect the pull-down cadence. Normal convolution-based matched filters do a poor job since there may be a lot of noise in the signal, motion may not be clear on pull-down frames, and the overall signal may have a wide range of motion and signal intensity (brightness). There are other problems associated with using a matched filter for signal detection other than being sensitive to the noise level in a signal. One problem is that it does not work well with impulse signals, because no matter what signal is used for the matched filter, a match will occur, so a matched filter will not be discriminating enough since there is not sufficient information in an impulse signal. The convolution-based matched filter performs better when looking for a signal with a particular shape, and not an impulse. In general, convolution-based matched filter performs poorly with signals that have impulses in them. Another problem with using matched filters is that they are sensitive to DC offsets. Typically, after convolving with a matched filter, a comparison with a threshold is done, and one DC offset may give a signal strength that is higher than a smaller DC offset.
Correlation-based matched filters are immune to DC offsets, signal gain, and dynamic range, and as such they provide better detection. They also work well on matching to impulse functions. However, a correlation function is difficult to implement as it involves computation of squares and square root functions requiring very high precision, and require a great amount of hardware to implement.
Also, conventional 3:2 pull-down circuits rely upon host software to compute Pearson's Correlation, which is a standard correlation function used in correlation-based match filters. Pearson's correlation uses the variance of a signal as an estimate of error in the denominator, which is the difficult to compute. Conventional 3:2 pull-down circuits do not have the capability to operate independently in the hardware due to the complexity of the computations.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.